with d as (
select
monthly as YM,
new_mrr + expansion_mrr - downgrade_mrr - churn_mrr as diff,
date(monthly, '+1 month') prev

from mrr_data
order by monthly
)

-- RECURSIVE dates(x) AS (
--   (select min(monthly) from mrr_data)
--   UNION ALL
--   SELECT date(x, '+1 month') FROM cnt WHERE x <= (select max(monthly) from mrr_data)
-- )

-- select * from dates;

SELECT YM,
       ifnull(SUM(diff) OVER (ORDER BY YM ROWS BETWEEN UNBOUNDED PRECEDING AND 1 preceding), 0) AS MRR 
FROM   d
ORDER BY YM